Reducing the noise of aircraft has represented for a few years now a substantial socio-economic challenge. Indeed, air traffic is constantly growing, whether in terms of the number of flights or by the size of the aircraft. This growth is however up against the harmonious integration of commercial aviation activity into the environment due to the noise pollution generated by the aircraft.
As such, the high sound levels emitted by the aircraft in take-off and approach phase are particularly inconveniencing for the citizens living close to airports and the authorities therefore are forcing aircraft manufacturers to reduce these noises.
On the other hand, the requirements of the passengers in terms of comfort are increasing, and the reduction of the engine noises in the cabin is a substantial factor in terms of improving this comfort.
Consequently, reducing acoustic emissions of aircraft is one of the priorities of aircraft manufacturers.
Reducing the noise of aircraft is however a scientific and technical challenge as the notion of noise entails complex phenomena which are still poorly known, such as psycho-acoustics which relates to the perception of noises by the human ear.
Noise is defined as any unpleasant and inconveniencing auditory sensation and any acoustic phenomenon producing this sensation.
The studies carried out in psycho-acoustics show that the sensation of the discomfort caused by a noise depends primarily on its purity, its intensity and its frequency.
The human auditory system is sensitive to frequencies ranging from 20 Hz to a maximum of about 20,000 Hz. However, it is known that the human ear does not have the same sensitivity for all of the audible frequencies and as such the relation between the perception of a pure noise and the intensity of this noise is not linear.
FIG. 1 shows as such isosonic curves of the human ear according to the standard ISO226:2003. These curves show a measurement of the sound pressure (in decibels A), according to the frequency, that a person perceives as a sound of the same level.
With regards to FIG. 1, it is observed that the human ear is less sensitive to the low frequencies. Consequently, the human brain accepts more easily, for the same amount of power, a low-frequency audible signal than a high- or medium-frequency audible signal. By way of example, a sound of 50 dB(A) and with a frequency of 1000 Hz produces an auditory sensation that is stronger than a sound of 50 dB(A) with a frequency of 100 Hz. Note in addition that the human ear is particularly sensitive to frequencies between 1000 Hz and 3000 Hz.
On the other hand, the human brain accepts more easily an audible signal emitted over a wide band of frequencies than pure or tonal noise. Indeed, studies carried out on many subjects show that the human ear perceives a sound as pleasant if the latter is comprised of a high number of frequencies.
Reducing the noise of an aircraft requires taking its motorisation into account: turboprop or turbojet.
In a turbojet, the various vaned wheels, such as the fan and the vaned wheels of the compressor or compressors, are ducted.
Many studies have been conducted in order to decrease the noises due to turbojet aircraft, which represent the majority of the world fleet. The noises emitted by the rotors of turbojets are particularly inconveniencing as their fundamental frequency is of a few thousand Hertz (of a magnitude of 1000-2000 Hz), frequencies to which the human ear is particularly sensible, as shown in FIG. 1.
As such, U.S. Pat. No. 5,966,525 relates to the acoustic radiation of a turbojet induced by geometrical imprecisions of the assembly of the vanes of its rotors. This additional source of noise is known under the name of Buzz Saw Noise (BSN).
Due to the fairing of the rotor, certain harmonics are propagated outside of the air duct wherein this rotor moves, while others are evanescent. The method described in this patent then consists in displacing the energy of the acoustic spectrum of the turbojet, and more particularly of its BSN component, in order to concentrate it in the non-propagative portion of the acoustic spectrum. For this purpose, the vanes of the rotor, having slight geometrical variations between them, are distributed regularly around the axis of the rotor and in a sinusoidal manner with a high period in order to concentrate the energy on harmonics of a low degree with an evanescent nature.
This technique, although it is effective, is however complex since it requires manufacturing different vanes, with slight geometrical variations.
The noise reduction techniques applied to turbojets cannot be applied to turboprops.
Indeed, the propeller of a turboprop is not ducted and as such all of the frequencies in the spectrum of its audible signal are transmitted. The same applies in the case of turbomachines with a couple of unducted contrarotating propellers, also referred to as “open rotor”.
In addition, the rotating speeds of a propeller of a turboprop or of an “open rotor” are substantially less than those of a rotor of the vaned wheels of a turbojet. Indeed, the fundamental frequency of the acoustic spectrum radiated by a propeller of this type is of a few hundred Hertz in such a way that the techniques that can reduce the noise of it are very different.
The fundamental frequency of the acoustic spectrum radiated by a propeller is easily calculated. In the case of a conventional propeller with n blades rotating at a rotating speed R (rpm), the propeller returns to an initial state for a rotation of 1/n revolutions. The period of the acoustic phenomenon is given by:
                              T          ⁢                                          ⁢          1                =                              (                          60              R                        )                    ·                      (                          1              /              n                        )                                              (        1        )            
and the frequency of the acoustic phenomenon is:f1=1/T1=(R·n)/60  (2)
In particular, the fundamental frequency of acoustic radiation of a propeller with 6 blades is f=RPM/10 since the propeller returns to its initial state every 1/6 revolutions.
The traction of a propeller is a determining parameter on the level of noise in db(A) that it radiates. Indeed, an increase of the traction is equivalent to an increase in the load of the propeller inducing a noise referred to as “load” or dipolar noise.
The increase of the traction in transonic configuration also induces an increase in the intensity of the shocks on the blade, and therefore of the shock noise referred to as “quadrupole”.
FIG. 2 shows a diagrammatical representation of the forces being exerted on an aerofoil of a propeller blade of an aircraft.
The traction vector T of the blade 1, directed according to the direction of the flight of the aircraft, is given by the projection of the total force Ft on the engine axis X, or axis of rotation of the propeller. The blade 1, of which the aerofoil is similar to that of an aircraft wing, has indeed a relative velocity W and is subjected to a lift force Fz, perpendicular to its relative velocity vector W and a drag force Fx, collinear to its relative velocity vector W.
The traction T delivered by a blade of a propeller is in particular linked to the pitch angle β of the blades by the intermediary of the angle of incidence α and of the angle φ that the relative velocity vector W of the aerofoil forms with the plane of rotation of the propeller Y. Indeed, the angles β, α and φ are linked by the following relationship:β=α+φ  (3)
Pitch angle means the angle between the chord of an aerofoil of a blade of the propeller and a plane perpendicular to the axis of rotation of the propeller. The chord is defined as the segment connecting the leading edge with the trailing edge of the blade.
A propeller blade comprises a blade root by means of which the blade is fixed on the hub of the propeller, as well as an aerodynamic portion extending radially towards the exterior in relation to the axis of rotation of the propeller from the root of the blade. The aerodynamic portion comprises a plurality of aerofoils between its radially inner end, referred to as base of the blade and located nearby the root of the blade, and its radially outer end, referred to as blade tip.
In this description, the aerofoil chosen to define the pitch angle is the aerofoil of the base of the blade.
The pitch angle of the set of blades of a propeller can be modified during the flight, in a controlled manner or automatically, in order to modify the traction supplied by the propeller.
Certain tests have been carried out in order to reduce the noise of an aft rotor of a helicopter. One solution tested consists in designing a rotor wherein the distribution of the blades around the axis of rotation is not regular, in order to reduce the interaction noise of the rotor with its supports. However, such a system is relatively complex and fragile and cannot be applied to a turbomachine propeller without considerably increasing the mass and the maintenance costs of the latter. This type of solution is therefore not optimal in the framework of a large-scale commercial operation.